Collaborative and Other Relationships - Summary of Activity Related to LEQEMBI Collaboration (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
LEQEMBI \| Alzheimer's collaboration revenue
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI	$ 70.9	$ 77.7
Expense reflected within statements of income	35.4	38.8
LEQEMBI \| Alzheimer's collaboration revenue \| Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income	88.3	82.3
Expense incurred by the collaboration	$ 176.8	$ 164.6
E2609 and BAN2401 \| Eisai
Collaborative arrangements and non-collaborative arrangement transactions
Total development expense incurred by the collaboration related to the advancement of LEQEMBI			$ 178.4	$ 255.6
Expense reflected within statements of income			89.2	127.8
E2609 and BAN2401 \| Eisai \| Selling, general and administrative
Collaborative arrangements and non-collaborative arrangement transactions
Expense reflected within statements of income			262.8	238.1
Expense incurred by the collaboration			$ 525.7	$ 476.2